1. Field of the Invention
The invention relates to the control of the machining of workpieces by a machine tool in which the tool and the workpiece rotate relatively to one another to produce a shaped profile on the workpiece.
2. Review of the Prior Art
In recent years, the computer control of machine tools has been widely developed. The computer produces a succession of digital signals which are used to position the tool to provide a required profile on the workpiece. In general, the computer calculates the required digital signals in advance and feeds them to a store where they are held and fed at required times to a conventional tool moving system which comprises a tool slide moved by a motor through a feed screw. It is not always necessary for there to be any tool position feedback and so the motor can be a stepping motor which is controlled by counting down a register containing a signal corresponding to the required number of steps of the motor to produce the required tool position.
Because of the comparatively low speed of relative rotation between the tool and the workpiece, and because of the absence of any tool feedback, the production of the digital signals can be readily handled by a computer of modest size. In addition, the inertia of the slide is sufficient to smooth out the stepped nature of the signal applied to the motor.
If, however, the relative speed of rotation of the tool and the workpiece is increased dramatically, and if the tool is required to alter its position within a revolution, an entirely different situation prevails. First, it is necessary to have a tool capable of being moved very rapidly indeed; that is a tool holder having a low inertia. Tool position feedback is necessary to ensure that the position of the tool alters accurately within each revolution. At these speeds, computers conventionally used will produce digital tool control signals at a rate which is insufficiently frequent to permit the tool holder to move in accordance with such signals; the tool holder would be required to have infinite acceleration between successive signals and this is plainly not possible. The computer thus does not have the capacity to produce signals at a frequency sufficient to allow the tool holder to make the required movements.
This lack of capacity of conventional computers produces the further problem that the computer is incapable of performing the required feedback control while maintaining the required high rotational speeds. Indeed, if the computer is required to produce digital tool control signals at a substantial number of positions within a revolution (as may be necessary to define many required profiles), the computer will generally be incapable of storing all the required digital signals for the machining operation.
In general, therefore, it has not heretofore been possible to machine complex profiles on a workpiece by computer numerical control at the rates required for commercial production.
Such machining has in general been performed by cam follower machines in which a shaped cam is contacted by a cam follower whose motion is transmitted to the tool. The production of the shaped cams is, however, both time consuming and expensive and the use of cams makes the process inflexible. In addition, the speed at which a cam follower can follow a cam is limited, since at high rotational speeds the follower may lift-off the cam so introducing inaccuracies into the workpiece.